The present invention relates to a photodiode for generating an electrical current upon receiving ultraviolet light. Further, the present invention relates to an ultraviolet sensor having the photodiode, and a method of producing the photodiode.
Patent Reference has disclosed a conventional ultraviolet sensor for detecting ultraviolet light. The conventional photodiode includes a P-type silicon substrate having two N-type areas. A P-type diffusion layer is formed in a surface layer of each of the N-type areas. An N-type diffusion layer is formed in an area including an interface of the P-type silicon substrate and the N-type area to face the P-type diffusion layer with the N-type area in between.
In the conventional ultraviolet sensor, depths of the P-type diffusion layers are adjusted to form two photodiodes of a lateral PN junction type. Due to a difference in the depths of the P-type diffusion layers, an amount of ultraviolet light reaching the PN junction below the P-type diffusion layers changes. Further, a photoelectric current of the photodiode having the P-type diffusion layer with a larger depth is subtracted from a photoelectric current of the photodiode having the P-type diffusion layer with a smaller depth to cancel out a photoelectric current of visible light, thereby detecting an intensity of ultraviolet light.
Patent Reference: Japanese Patent Publication No. 2007-67331
In general, ultraviolet light is categorized into three types of light, i.e., long wave ultraviolet light (UV-A light, a wavelength between 320 nm and 400 nm); intermediate wave ultraviolet light (UV-B light, a wavelength between 280 nm and 320 nm); and short wave ultraviolet light (UV-C light, a wavelength of less than 280 nm). More than 90% of ultraviolet light reaching the ground level is formed of the UV-A light, and the remaining portion is formed of the UV-B light. The UV-C light is mostly absorbed in the ozone layer, and does not reach the ground level.
The three types of ultraviolet light have different effects on a human body or environment depending on wavelengths thereof. More specifically, the UV-B light induces skin inflammation and skin cancer. Accordingly, it has been required to develop an ultraviolet sensor capable of effectively separating and detecting the UV-B light.
In the conventional ultraviolet sensor, it is possible to detect a total amount of ultraviolet light in an ultraviolet light region less than 400 nm. However, it is difficult to separate and detect ultraviolet light in a specific wavelength region.
As described above, in the conventional ultraviolet sensor, the depths of the P-type diffusion layers are adjusted for changing the photoelectric currents outputting from the two photodiodes. A different between the photoelectric currents is determined through a calculation, thereby detecting an intensity of ultraviolet light in the ultraviolet light region. Accordingly, when the depths of the P-type diffusion layers are varied upon forming the P-type diffusion layers, an amount of ultraviolet light absorbed in the P-type diffusion layers varies. Accordingly, it is difficult to produce the conventional ultraviolet sensor with stable property.
In view of the problems described above, an object of the present invention is to provide a photodiode and a method of producing the photodiode capable of solving the problems of the conventional ultraviolet sensor. In the present invention, it is possible to effectively separate and detect ultraviolet light in the wavelength region of the UV-B light.
Further objects and advantages of the invention will be apparent from the following description of the invention.